The invention is in the field of using an acousto-optic (AO) crystal to provide a constant frequency offset to a portion of the laser transmitter power for injection locking a separate laser oscillator. A problem exists in that heterodyne laser radar sensors required a stabilized local oscillator (LO). The LO must be at a constant offset frequency from the transmitted frequency since it is the frequency differences between the LO and the return signal, which consists of the transmitted frequency shifted by the doppler frequency f.sub.d of the target, that is processed when the two frequencies are mixed at an optical detector. If the LO is not stable in frequency offset with the transmitted frequency, erroneous measurements will be made. The present method insures LO stability in frequency offset.
Previous methods use either AO crystals for offset-homodyne or a separate locked laser to achieve the heterodyne effect. The use of AO crystals introduce a power reduction of 20% or more for each crystal used. The AO crystal may be placed in the LO leg to avoid power loss but an undesirable component of the LO radiation will pass through the AO crystal without the necessary frequency shift severely limiting the detection sensitivity. Also, it is very difficult to maintain a stable frequency offset with the transmitter when using a separate locked laser.